It sounds like your washing machine is sucking water and air from your toilets so it can complete drain cycle. That is the purpose of the venting. Have you always had this problem or did it recently begin? Chances are if you have always had it, there is a problem with the way your venting was designed. If it is a recent problem, or has gotten worse recently, I think you have a blockage in the vent pipe that is preventing the main vent from providing air to the the flush cycle.
JJ
|